Excess air in combustion – purpose and rationale To achieve complete and rapid combustion in practical burners and engines, should some air in excess of the theoretical (stoichiometric) requirement be supplied?

Introduction / Context:
Stoichiometric air ensures exact oxygen for complete reaction with fuel. In real equipment, mixing limitations and diffusion constraints mean that theoretical air is often insufficient to guarantee complete burnout and acceptable emissions. This question addresses the engineering practice of providing excess air.

Given Data / Assumptions:

• Practical combustors (boilers, furnaces, IC engines, burners).
• Finite mixing rates and non-uniformity of local fuel–air ratios.
• Desire for complete combustion (minimal CO, unburned hydrocarbons, soot).

Concept / Approach:

Excess air increases the availability of oxygen in regions where mixing is imperfect, promoting the final oxidation steps from CO to CO2 and from H2/HC to H2O and CO2. While too much excess air lowers flame temperature and thermal efficiency (due to nitrogen ballast and stack losses), a moderate percentage is standard practice to balance efficiency, stability, and emissions. Typical ranges vary by fuel and device (e.g., 5–10% for gas burners, higher for oil/solid fuels).

Step-by-Step Solution:

Verification / Alternative check:

Flue-gas oxygen analyzers show residual O2 when excess air is used; CO and unburned HC drop as excess air rises from zero to an optimal value before efficiency penalties dominate.

Why Other Options Are Wrong:

“No” disregards real mixing constraints; categorical statements for specific fuels are incorrect since most systems benefit from some excess air.Claiming excess air reduces flame speed to zero is physically false; it modestly reduces temperature and speed but does not extinguish combustion unless extremely high.

Common Pitfalls:

Assuming more excess air is always better; too much raises stack losses and may cause instability. Optimal excess air is a controlled compromise.

Final Answer:

Yes